On 2023-12-30T23:07:54+01:00, christoph.thiede@student.hpi.uni-potsdam.de wrote:
Hi Jaromir,
I found a breaking change in the new behavior of Context>>#return:from: while using the TraceDebugger:
In the past we could say:
c := [2+3] asContext. [c] whileNotNil: [c := c step].
With your change, the script runs forever because the last step does not answer nil as before but activates a new #cannotReturn:.
This behavior seems not be expected anywhere in the trunk (if my first search was complete), and you are right that the new behavior aligns closer to the VM behavior. Still, the old code seemed to explicitly intend this - see the "newTop ifNotNil:" at the bottom of the method.
I wonder whether we should keep this. For me it is not a big deal; I can just change my script like this:
c := [2+3] asContext. [c sender isNil and: [c willReturn]] whileNotNil: [c := c step].
#whileFalse:, not #whileNotNil:, of course:
c := [2+3] asContext. [c sender isNil and: [c willReturn]] whileFalse: [c := c step].
I just wonder whether this could a breaking or unintended change for anything else. For [^2] ensure: [] it would not be a big deal, we could just change the check in question to (aSender isDead or: [newTop notNil and: [newTop isDead]]) ifTrue:. I am tending against restoring the old behavior, but I am unsure. What is your opinion on this?
Best, Christoph
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On 2023-12-30T21:13:37+00:00, mail(a)jaromir.net wrote:
nit: You mixed up the order of arguments for #assert:equals:
oops, sorry :) It happens to me all the time; I've never actually understood why the strange, almost Yodaesque, order... as if you asked in English:
"Make sure 18 is his age."
Thanks, Jaromir
On 30-Dec-23 9:13:56 PM, christoph.thiede(a)student.hpi.uni-potsdam.de wrote:
nit: You mixed up the order of arguments for #assert:equals: (it is assert: expected equals: actual) and could have used it in the final assert again, but that's clearly no reason to hold back a useful test. ;-) Merged, thanks! :-)
Best, Christoph
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On 2023-12-30T17:33:08+00:00, mail(a)jaromir.net wrote:
Hi Christoph,
Thanks for merging the fixes; I've just sent another test in KernelTests-jar.448 to complement them.
Please take a look and if ok I'd appreciate it if you could merge it
as
well.
Best regards and Happy New Year to you too! Jaromir
On 30-Dec-23 6:15:25 PM,
christoph.thiede(a)student.hpi.uni-potsdam.de
wrote:
Hi Jaromir, hi all,
finally I have found the time to review these suggestions. Kernel-jar.1537, Kernel-jar.1538, and Kernel-jar.1539 look excellent
to
me as well. Clear, straightforward, useful. :-) I have merged them
into
the trunk via Kernel-ct.1545.
Regarding DebuggerTests>>test16HandleSimulationError, I have patched
it
via ToolsTests-ct.125. Nothing to rack your brains over:
"thisContext
pc: nil" just mimicks any kind of unhandled error inside the
simulator
- since we now gently handle this via #cannotReturn:, I just
replaced
it with "thisContext pc: false". :-) Sorry for not clarifying that earlier and letting you speculate.
Thanks for your work, and I already wish you a happy new year!
Best, Christoph
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On 2023-11-29T13:31:09+00:00, mail(a)jaromir.net wrote:
Hi Marcel,
> [myself] whether the patch would have been necessary should the #return:from: had been fixed then
Nonsense, I just mixed it up with another issue :)
On 29-Nov-23 1:51:21 PM, "Jaromir Matas" <mail(a)jaromir.net>
wrote:
>Thanks Marcel! This test somehow slipped my attention :) > >The test can no longer work as is. It takes advantage of the
erroneous
>behavior of #return:from: in the sense that if you simulate > > thisContext pc: nil > >it'll happily return to a dead context (i.e. to thisContext from
#pc:
>nil context) - which is not what the VM does during runtime. It
should
>immediately raise an illegal return exception not only during
runtime
>but also during simulation. > >The test mentions a patch for an infinite debugger chain >(http://forum.world.st/I-broke-the-debugger-td5110752.html). I
wonder
>whether the problem could have something to do with this
simulation
bug
>in return:from:; and a terrible idea occurred to me whether the
patch
>would have been necessary should the #return:from: had been
fixed
then
>;O > >We may potentially come up with more examples like this, even in
the
>trunk, where the bug from #return:from: propagated and was taken >advantage of. I've found and fixed #runUntilErrorOrReturnFrom:
but
more
>can still be surviving undetected... > >I'd place the test into #expectedFailures for now but maybe it's
time
>to remove it; Christoph should decide :) > >Thanks again, >Jaromir > > >On 29-Nov-23 10:28:38 AM, "Taeumel, Marcel via Squeak-dev" ><squeak-dev(a)lists.squeakfoundation.org> wrote: > >>Hi Jaromir -- >> >>Looks good. Still, what about that #test16HandleSimulationError
now?
>>:-) It is failing with your changes ... how would you adapt it? >> >> >> >>Best, >>Marcel >>>Am 28.11.2023 01:29:39 schrieb Jaromir Matas
<mail(a)jaromir.net>:
>>> >>>Hi Eliot, Marcel, all, >>> >>>I've sent a fix Kernel-jar.1539 to the Inbox that solves the >>>remaining bit of the chain of bugs described in the previous
post.
>>>All tests are green now and I think the root cause has been
found
and
>>>fixed. >>> >>>In this last bit I've created a version of stepToCallee that
would
>>>identify a potential illegal return to a nil sender and avoid
it.
>>> >>>Now this example can be debugged without any problems: >>> >>>[[self halt. ^ 1] on: BlockCannotReturn do: #resume ] fork >>> >>>If you're happy with the solution in Kernel-jar.1539, >>>Kernel-jar.1538, Kernel-jar.1537 and the test in
KernelTests-jar.447,
>>>could you please double-check and merge, please? (And remove >>>Kernel-mt.1534 and Tools-jar.1240 from the Inbox) >>> >>>Best, >>>Jaromir >>> >>> >>> >>>On 27-Nov-23 12:09:37 AM, "Jaromir Matas" <mail(a)jaromir.net>
wrote:
>>> >>>>Hi Eliot, Christoph, all >>>> >>>>It looks like there are some more skeletons in the closet :/ >>>> >>>>If you run this example >>>> >>>>[[self halt. ^ 1] on: BlockCannotReturn do: [:ex | ex resume]
]
fork
>>>> >>>>and step over halt and then step over ^1 you get a
nonsensical
error
>>>>as a result of decoding nil as an instruction. >>>> >>>>It turns out that the root cause is in the #return:from:
method:
it
>>>>only checks whether aSender is dead but ignores the
possibility
that
>>>>aSender sender may be nil or dead in which cases the VM also >>>>responds with sending #cannotReturn, hence I assume the
simulator
>>>>should do the same. In addition, the VM nills the pc in such >>>>scenario, so I added the same functionality here too: >>>> >>>>Context >> return: value from: aSender >>>> "For simulation. Roll back self to aSender and return value >>>>from it. Execute any unwind blocks on the way. ASSUMES
aSender is
>>>>a sender of self" >>>> >>>> | newTop | >>>> newTop := aSender sender. >>>> (aSender isDead or: [newTop isNil or: [newTop isDead]])
ifTrue:
>>>> "<--------- this is extended ------" >>>> [^self pc: nil; send: #cannotReturn: to: self with: >>>>{value}]. "<------ pc: nil is added ----" >>>> (self findNextUnwindContextUpTo: newTop) ifNotNil: >>>> "Send #aboutToReturn:through: with nil as the second >>>>argument to avoid this bug: >>>> Cannot #stepOver '^2' in example '[^2] ensure: []'. >>>> See
>http://lists.squeakfoundation.org/pipermail/squeak-dev/2022-June/220975.html
>http://lists.squeakfoundation.org/pipermail/squeak-dev/2022-June/220975.html" >>>> [^self send: #aboutToReturn:through: to: self with: {value. >>>>nil}]. >>>> self releaseTo: newTop. >>>> newTop ifNotNil: [newTop push: value]. >>>> ^newTop >>>> >>>>In order for this to work #cannotReturn: has to be modified
as in
>>>>Kernel-jar.1537: >>>> >>>>Context >> cannotReturn: result >>>> >>>> closureOrNil ifNotNil: [^ self cannotReturn: result to: self >>>>home sender]. >>>> self error: 'Computation has been terminated!' >>>>"<----------- this has to be an Error -----" >>>> >>>>Then it almost works except when you keep stepping over in
the
>>>>example above, you get an MNU error on `self previousPc` in >>>>#cannotReturn:to:` with your solution of the VM crash. If you
don't
>>>>mind I've amended your solution and added the final context
where
>>>>the computation couldn't return along with the pc: >>>> >>>>Context >> cannotReturn: result to: homeContext >>>> "The receiver tried to return result to homeContext that
cannot
>>>>be returned from. >>>> Capture the return context/pc in a BlockCannotReturn. Nil
the pc
>>>>to prevent repeat >>>> attempts and/or invalid continuation. Answer the result of >>>>raising the exception." >>>> >>>> | exception previousPc | >>>> exception := BlockCannotReturn new. >>>> previousPc := pc ifNotNil: [self previousPc]. "<----- here's
a
>>>>fix ----" >>>> exception >>>> result: result; >>>> deadHome: homeContext; >>>> finalContext: self; "<----- here's the new state, if >>>>that's fine ----" >>>> pc: previousPc. >>>> pc := nil. >>>> ^exception signal >>>> >>>>Unfortunately, this is still not the end of the story: there
are
>>>>situations where #runUntilErrorOrReturnFrom: places the two
guard
>>>>contexts below the bottom context. And that is a problem
because
>>>>when the method tries to remove the two guard contexts before >>>>returning at the end it uses #stepToCalee to do the job but
this
>>>>unforotunately was (ab)using the above bug in #return:from: -
I'll
>>>>explain: #return:from: didn't check whether aSender sender
was
nil
>>>>and as a result it allowed to simulate a return to a "nil
context"
>>>>which was then (ab)used in the clean-up via #stepToCalee in
the
>>>>#runUntilErrorOrReturnFrom:. >>>> >>>>When I fixed the #return:from: bug, the
#runUntilErrorOrReturnFrom:
>>>>cleanup of the guard contexts no longer works in that very
special
>>>>case where the guard contexts are below the bottom context.
There's
>>>>one case where this is being used: #terminateAggresively by >>>>Christoph. >>>> >>>>If I'm right with this analysis, the
#runUntilErrorOrReturnFrom:
>>>>should get fixed too but I'll be away now for a few days and
I
won't
>>>>be able to respond. If you or Christoph had a chance to take
a
look
>>>>at Kernel-jar.1538 and Kernel-jar.1537 I'd be very grateful.
I
hope
>>>>this super long message at least makes some sense :) >>>>Best, >>>>Jaromir >>>> >>>>[1] Kernel-jar.1538, Kernel-jar.1537 >>>>[2] KernelTests-jar.447 >>>> >>>> >>>>PS: Christoph, >>>> >>>>With Kernel-jar.1538 + Kernel-jar.1537 your example >>>> >>>>process := >>>> [(c := thisContext) pc: nil. >>>> 2+3] newProcess. >>>>process runUntil: [:ctx | ctx selector = #cannotReturn:]. >>>>self assert: process suspendedContext sender sender = c. >>>>self assert: process suspendedContext arguments = {c}. >>>> >>>>works fine, I've just corrected your first assert. >>>> >>>> >>>> >>>> >>>> >>>>On 21-Nov-23 6:40:32 PM, "Eliot Miranda"
<eliot.miranda(a)gmail.com>
>>>>wrote: >>>> >>>>>Hi Jaromir, >>>>> >>>>>>On Nov 20, 2023, at 11:51 PM, Jaromir Matas
<mail(a)jaromir.net>
>>>>>>wrote: >>>>>> >>>>>> >>>>>>Hi Eliot, >>>>>>Very elegant! Now I finally got what you meant exactly :)
Thanks.
>>>>>> >>>>>>Two questions: >>>>>>1. in order for the enclosed test to work I'd need an Error >>>>>>instead of Processor debugWithTitle:full: call in
#cannotReturn:.
>>>>>>Otherwise I don't know how to catch a plain invocation of
the
>>>>>>Debugger: >>>>>> >>>>>>cannotReturn: result >>>>>> >>>>>> closureOrNil ifNotNil: [^ self cannotReturn: result to:
self
>>>>>>home sender]. >>>>>> self error: 'Computation has been terminated!' >>>>> >>>>>Much nicer. >>>>> >>>>>>2. We are capturing a pc of self which is completely
different
>>>>>>context from homeContext indeed. >>>>> >>>>>Right. The return is attempted from a specific return
bytecode
in a
>>>>>specific block. This is the coordinate of the return that
cannot
be
>>>>>made. This is the relevant point of origin of the cannot
return
>>>>>exception. >>>>> >>>>>Why the return fails is another matter: >>>>>- the home context’s sender is a dead context (cannot be
resumed)
>>>>>- the home context’s sender is nil (home already returned
from)
>>>>>- the block activation’s home is nil rather than a context
(should
>>>>>not happen) >>>>> >>>>>But in all these cases the pc of the home context is
immaterial.
>>>>>The hike is being returned through/from, rather than from;
the
>>>>>home’s pc is not relevant. >>>>> >>>>>>Maybe we could capture self in the exception too to make it
more
>>>>>>clear/explicit what is going on: what context the captured
pc
is
>>>>>>actually associated with. Just a thought... >>>>> >>>>>Yes, I like that. I also like the idea of somehow passing
the
>>>>>block activation’s pc to the debugger so that the relevant
return
>>>>>expression is highlighted in the debugger. >>>>> >>>>>> >>>>>>Thanks again, >>>>>>Jaromir >>>>> >>>>>You’re welcome. I love working in this part of the system.
Thanks
>>>>>for dragging me there. I’m in a slump right now and
appreciate
the
>>>>>fellowship. >>>>> >>>>>>------ Original Message ------ >>>>>>From "Eliot Miranda" <eliot.miranda(a)gmail.com> >>>>>>To "Jaromir Matas" <mail(a)jaromir.net> >>>>>>Cc squeak-dev(a)lists.squeakfoundation.org >>>>>>Date 11/21/2023 2:17:21 AM >>>>>>Subject Re: Re[2]: [squeak-dev] Re: Resuming on
BlockCannotReturn
>>>>>>exception >>>>>> >>>>>>>Hi Jaromir, >>>>>>> >>>>>>> see Kernel-eem.1535 for what I was suggesting. This
example
>>>>>>>now has an exception with the right pc value in it: >>>>>>> >>>>>>>[[^1] on: BlockCannotReturn do: [:ex| ex pc inspect. ex
resume]]
>>>>>>>fork >>>>>>> >>>>>>>The fix is simply >>>>>>> >>>>>>>Context>>cannotReturn: result to: homeContext >>>>>>> "The receiver tried to return result to homeContext that >>>>>>>cannot be returned from. >>>>>>> Capture the return pc in a BlockCannotReturn. Nil the pc
to
>>>>>>>prevent repeat >>>>>>> attempts and/or invalid continuation. Answer the result
of
>>>>>>>raising the exception." >>>>>>> >>>>>>> | exception | >>>>>>> exception := BlockCannotReturn new. >>>>>>> exception >>>>>>> result: result; >>>>>>> deadHome: homeContext; >>>>>>> pc: self previousPc. >>>>>>> pc := nil. >>>>>>> ^exception signal >>>>>>> >>>>>>> >>>>>>>The VM crash is now avoided. The debugger displays the
method,
>>>>>>>but does not highlight the offending pc, which is no big
deal.
A
>>>>>>>suitable defaultHandler for B lockCannotReturn may be able
to
get
>>>>>>>the debugger to highlight correctly on opening. Try the >>>>>>>following examples: >>>>>>> >>>>>>>[[^1] on: BlockCannotReturn do: #resume] fork. >>>>>>> >>>>>>>[[^1] on: BlockCannotReturn do: [:ex| ex pc inspect. ex
resume]]
>>>>>>>fork >>>>>>> >>>>>>>[[^1] value] fork. >>>>>>> >>>>>>>They al; seem to behave perfectly acceptably to me. Does
this
>>>>>>>fix work for you? >>>>>>> >>>>>>>On Fri, Nov 17, 2023 at 3:14 PM Jaromir Matas
<mail(a)jaromir.net>
>>>>>>>wrote: >>>>>>>>Hi Eliot, >>>>>>>> >>>>>>>>How about to nil the pc just before making the return: >>>>>>>>``` >>>>>>>>Context >> #cannotReturn: result >>>>>>>> >>>>>>>> self push: self pc. "backup the pc for the sake of >>>>>>>>debugging" >>>>>>>> closureOrNil ifNotNil: [^self cannotReturn: result to:
self
>>>>>>>>home sender; pc: nil]. >>>>>>>> Processor debugWithTitle: 'Computation has been
terminated!'
>>>>>>>>translated full: false >>>>>>>>``` >>>>>>>>The nilled pc should not even potentially interfere with
the
>>>>>>>>#isDead now. >>>>>>>> >>>>>>>>I hope this is at least a step in the right direction :) >>>>>>>> >>>>>>>>However, there's still a problem when debugging the
resumption
>>>>>>>>of #cannotReturn because the encoders expect a reasonable
index.
>>>>>>>>I haven't figured out yet where to place a nil check -
#step,
>>>>>>>>#stepToSendOrReturn... ? >>>>>>>> >>>>>>>>Thanks again, >>>>>>>>Jaromir >>>>>>>> >>>>>>>> >>>>>>>>------ Original Message ------ >>>>>>>>From "Eliot Miranda" <eliot.miranda(a)gmail.com> >>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net> >>>>>>>>Date 11/17/2023 8:36:50 PM >>>>>>>>Subject Re: [squeak-dev] Re: Resuming on
BlockCannotReturn
>>>>>>>>exception >>>>>>>> >>>>>>>>>Hi Jaromir, >>>>>>>>> >>>>>>>>>>On Nov 17, 2023, at 7:05 AM, Jaromir Matas
<mail(a)jaromir.net>
>>>>>>>>>>wrote: >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>Eliot, hi again, >>>>>>>>>> >>>>>>>>>>Please disregard my previous comment about nilling the >>>>>>>>>>contexts that have returned. We are indeed talking
about
the
>>>>>>>>>>context directly under the #cannotReturn context which
is
>>>>>>>>>>totally different from the home context in another
thread
>>>>>>>>>>that's gone. >>>>>>>>>> >>>>>>>>>>I may still be confused but would nilling the pc of the >>>>>>>>>>context directly under the cannotReturn context help?
Here's
>>>>>>>>>>what I mean: >>>>>>>>>>``` >>>>>>>>>>Context >> #cannotReturn: result >>>>>>>>>> >>>>>>>>>> closureOrNil ifNotNil: [^self pc: nil; cannotReturn: >>>>>>>>>>result to: self home sender]. >>>>>>>>>> Processor debugWithTitle: 'Computation has been >>>>>>>>>>terminated!' translated full: false. >>>>>>>>>>``` >>>>>>>>>>Instead of crashing the VM invokes the debugger with
the
>>>>>>>>>>'Computation has been terminated!' message. >>>>>>>>>> >>>>>>>>>>Does this make sense? >>>>>>>>> >>>>>>>>>Nearly. But it loses the information on what the pc
actually
>>>>>>>>>is, and that’s potentially vital information. So IMO the
ox
>>>>>>>>>should only be nilled between the BlockCannotReturn
exception
>>>>>>>>>being created and raised. >>>>>>>>> >>>>>>>>>[But if you try this don’t be surprised if it causes a
few
>>>>>>>>>temporary problems. It looks to me that without a little >>>>>>>>>refactoring this could easily cause an infinite
recursion
>>>>>>>>>around the sending of isDead. I’m sure you’ll be able to
fix
>>>>>>>>>the code to work correctly] >>>>>>>>> >>>>>>>>>>Thanks, >>>>>>>>>>Jaromir >>>>>>>>>> >>>>>>>>>> >>>>>>>>>>------ Original Message ------ >>>>>>>>>>From "Jaromir Matas" <mail(a)jaromir.net> >>>>>>>>>>To "Eliot Miranda" <eliot.miranda(a)gmail.com>; "The >>>>>>>>>>general-purpose Squeak developers list" >>>>>>>>>><squeak-dev(a)lists.squeakfoundation.org> >>>>>>>>>>Date 11/17/2023 10:15:17 AM >>>>>>>>>>Subject [squeak-dev] Re: Resuming on BlockCannotReturn >>>>>>>>>>exception >>>>>>>>>> >>>>>>>>>>>Hi Eliot, >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>>------ Original Message ------ >>>>>>>>>>>From "Eliot Miranda" <eliot.miranda(a)gmail.com> >>>>>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net> >>>>>>>>>>>Cc "The general-purpose Squeak developers list" >>>>>>>>>>><squeak-dev(a)lists.squeakfoundation.org> >>>>>>>>>>>Date 11/16/2023 11:52:45 PM >>>>>>>>>>>Subject Re: Re[2]: [squeak-dev] Re: Resuming on >>>>>>>>>>>BlockCannotReturn exception >>>>>>>>>>> >>>>>>>>>>>>Hi Jaromir, >>>>>>>>>>>> >>>>>>>>>>>>On Thu, Nov 16, 2023 at 2:22 PM Jaromir Matas >>>>>>>>>>>><mail(a)jaromir.net> wrote: >>>>>>>>>>>>>Hi Nicolas, Eliot, >>>>>>>>>>>>> >>>>>>>>>>>>>here's what I understand is happening (see the
enclosed
>>>>>>>>>>>>>screenshot): >>>>>>>>>>>>> >>>>>>>>>>>>>1) we fork a new process to evaluate [^1] >>>>>>>>>>>>>2) the new process evaluates [^1] which means
instruction
>>>>>>>>>>>>>18 is being evaluated, hence pc points to
instruction 19
>>>>>>>>>>>>>now >>>>>>>>>>>>>3) however, the home context where ^1 should return
to
is
>>>>>>>>>>>>>gone by this time (the process that executed the
fork
has
>>>>>>>>>>>>>already returned - notice the two up arrows in the
debugger
>>>>>>>>>>>>>screenshot) >>>>>>>>>>>>>4) the VM can't finish the instruction and returns
control
>>>>>>>>>>>>>to the image via placing the #cannotReturn: context
on
top
>>>>>>>>>>>>>of the [^1] context >>>>>>>>>>>>>5) #cannotReturn: evaluation results in signalling
the
BCR
>>>>>>>>>>>>>exception which is then handled by the #resume
handler
>>>>>>>>>>>>> (in our debugged case the [:ex | self halt. ex
resume]
>>>>>>>>>>>>>handler) >>>>>>>>>>>>>6) ex resume is evaluated, however, this means
requesting
>>>>>>>>>>>>>the VM to evaluate instruction 19 of the [^1]
context -
>>>>>>>>>>>>>which is past the last instruction of the context
and
the
>>>>>>>>>>>>>crash ensues >>>>>>>>>>>>> >>>>>>>>>>>>>I wonder whether such situations could/should be
prevented
>>>>>>>>>>>>>inside the VM or whether such an expectation is
wrong
for
>>>>>>>>>>>>>some reason. >>>>>>>>>>>> >>>>>>>>>>>>As Nicolas says, IMO this is best done at the image
level.
>>>>>>>>>>>> >>>>>>>>>>>>It could be prevented in the VM, but at great cost,
and
only
>>>>>>>>>>>>partially. The performance issue is that the last
bytecode
>>>>>>>>>>>>in a method is not marked in any way, and that to
determine
>>>>>>>>>>>>the last bytecode the bytecodes must be symbolically >>>>>>>>>>>>evaluated from the start of the method. See
implementors
of
>>>>>>>>>>>>endPC at the image level (which defer to the method
trailer)
>>>>>>>>>>>>and implementors of endPCOf: in the VMMaker code.
Doing
this
>>>>>>>>>>>>every time execution commences is prohibitively
expensive.
>>>>>>>>>>>>The "only partially" issue is that following the
return
>>>>>>>>>>>>instruction may be other valid bytecodes, but these
are
not
>>>>>>>>>>>>a continuation. >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>Consider the following code in some block: >>>>>>>>>>>> [self expression ifTrue: >>>>>>>>>>>> [^1]. >>>>>>>>>>>> ^2 >>>>>>>>>>>> >>>>>>>>>>>>The bytecodes for this are >>>>>>>>>>>> pushReceiver >>>>>>>>>>>> send #expression >>>>>>>>>>>> jumpFalse L1 >>>>>>>>>>>> push 1 >>>>>>>>>>>> methodReturnTop >>>>>>>>>>>>L1 >>>>>>>>>>>> push 2 >>>>>>>>>>>> methodReturnTop >>>>>>>>>>>> >>>>>>>>>>>>Clearly if expression is true these should be *no* >>>>>>>>>>>>continuation in which ^2 is executed. >>>>>>>>>>> >>>>>>>>>>>Well, in that case there's a bug because the
computation
in
>>>>>>>>>>>the following example shouldn't continue past the [^1]
block
>>>>>>>>>>>but it silently does: >>>>>>>>>>>`[[true ifTrue: [^ 1]] on: BlockCannotReturn do:
#resume ]
>>>>>>>>>>>fork` >>>>>>>>>>> >>>>>>>>>>>The bytecodes are >>>>>>>>>>> push true >>>>>>>>>>> jumpFalse L1 >>>>>>>>>>> push 1 >>>>>>>>>>> returnTop >>>>>>>>>>>L1 >>>>>>>>>>> push nil >>>>>>>>>>> blockReturn >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>So even if the VM did try and detect whether the
return
was
>>>>>>>>>>>>at the last block method, it would only work for
special
>>>>>>>>>>>>cases. >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>It seems to me the issue is simply that the context
that
>>>>>>>>>>>>cannot be returned from should be marked as dead (see >>>>>>>>>>>>Context>>isDead) by setting its pc to nil at some
point,
>>>>>>>>>>>>presumably after copying the actual return pc into
the
>>>>>>>>>>>>BlockCannotReturn exception, to avoid ever trying to
resume
>>>>>>>>>>>>the context. >>>>>>>>>>> >>>>>>>>>>>Does this mean, in other words, that every context
that
>>>>>>>>>>>returns should nil its pc to avoid being "wrongly" >>>>>>>>>>>reused/executed in the future, which concerns
primarily
those
>>>>>>>>>>>being referenced somewhere hence potentially
executable in
>>>>>>>>>>>the future, is that right? >>>>>>>>>>>Hypothetical question: would nilling the pc during
returns
>>>>>>>>>>>"fix" the example? >>>>>>>>>>>Thanks a lot for helping me understand this. >>>>>>>>>>>Best, >>>>>>>>>>>Jaromir >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>> >>>>>>>>>>>>>Thanks, >>>>>>>>>>>>>Jaromir >>>>>>>>>>>>> >>>>>>>>>>>>><bdxuqalu.png> >>>>>>>>>>>>> >>>>>>>>>>>>>------ Original Message ------ >>>>>>>>>>>>>From "Eliot Miranda" <eliot.miranda(a)gmail.com> >>>>>>>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net>; "The
general-purpose
>>>>>>>>>>>>>Squeak developers list" >>>>>>>>>>>>><squeak-dev(a)lists.squeakfoundation.org> >>>>>>>>>>>>>Date 11/16/2023 6:48:43 PM >>>>>>>>>>>>>Subject Re: [squeak-dev] Re: Resuming on
BlockCannotReturn
>>>>>>>>>>>>>exception >>>>>>>>>>>>> >>>>>>>>>>>>>>Hi Jaromir, >>>>>>>>>>>>>> >>>>>>>>>>>>>>>On Nov 16, 2023, at 3:23 AM, Jaromir Matas >>>>>>>>>>>>>>><mail(a)jaromir.net> wrote: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>Hi Nicloas, >>>>>>>>>>>>>>>No no, I don't have any practical scenario in
mind,
I'm
>>>>>>>>>>>>>>>just trying to understand why the VM is
implemented
like
>>>>>>>>>>>>>>>this, whether there were a reason to leave this >>>>>>>>>>>>>>>possibility of a crash, e.g. it would slow down
the VM
to
>>>>>>>>>>>>>>>try to prevent such a dumb situation (who would
resume
>>>>>>>>>>>>>>>from BCR in his right mind? :) ) - or perhaps I
have
>>>>>>>>>>>>>>>overlooked some good reason to even keep this
behavior
in
>>>>>>>>>>>>>>>the VM. That's all. >>>>>>>>>>>>>> >>>>>>>>>>>>>>Let’s first understand what’s really happening.
Presumably
>>>>>>>>>>>>>>at tone point a context is resumed those pc is
already
at
>>>>>>>>>>>>>>the block return bytecode (effectively, because it
crashes
>>>>>>>>>>>>>>in JITted code, but I bet the stack vm will crash
also,
>>>>>>>>>>>>>>but not as cleanly - it will try and execute the
bytes
in
>>>>>>>>>>>>>>the encoded method trailer). So which method
actually
>>>>>>>>>>>>>>sends resume, and to what, and what state is
resume’s
>>>>>>>>>>>>>>receiver when resume is sent? >>>>>>>>>>>>>> >>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>Thanks for your reply. >>>>>>>>>>>>>>>Regards, >>>>>>>>>>>>>>>Jaromir >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>------ Original Message ------ >>>>>>>>>>>>>>>From "Nicolas Cellier" >>>>>>>>>>>>>>><nicolas.cellier.aka.nice(a)gmail.com> >>>>>>>>>>>>>>>To "Jaromir Matas" <mail(a)jaromir.net>; "The >>>>>>>>>>>>>>>general-purpose Squeak developers list" >>>>>>>>>>>>>>><squeak-dev(a)lists.squeakfoundation.org> >>>>>>>>>>>>>>>Date 11/16/2023 7:20:20 AM >>>>>>>>>>>>>>>Subject Re: [squeak-dev] Resuming on
BlockCannotReturn
>>>>>>>>>>>>>>>exception >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>Hi Jaromir, >>>>>>>>>>>>>>>>Is there a scenario where it would make sense to
resume
>>>>>>>>>>>>>>>>a BlockCannotReturn? >>>>>>>>>>>>>>>>If not, I would suggest to protect at image side
and
>>>>>>>>>>>>>>>>override #resume. >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>Le mer. 15 nov. 2023, 23:42, Jaromir Matas >>>>>>>>>>>>>>>><mail(a)jaromir.net> a écrit : >>>>>>>>>>>>>>>>>Hi Eliot, Christoph, All, >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>It's known the following example crashes the VM.
Is
>>>>>>>>>>>>>>>>>this an intended behavior or a "tolerated bug"? >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>`[[^ 1] on: BlockCannotReturn do: #resume] fork` >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>I understand why it crashes: the non-local
return
has
>>>>>>>>>>>>>>>>>nowhere to return to and so resuming the
computation
>>>>>>>>>>>>>>>>>leads to a crash. But why not raise another BCR >>>>>>>>>>>>>>>>>exception to prevent the crash? Potential
infinite
>>>>>>>>>>>>>>>>>loop? Perhaps I'm just missing the purpose of
this
>>>>>>>>>>>>>>>>>behavior... >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>Thanks for an explanation. >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>Best, >>>>>>>>>>>>>>>>>Jaromir >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>-- >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>Jaromir Matas >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>-- >>>>>>>>>>>>_,,,^..^,,,_ >>>>>>>>>>>>best, Eliot >>>>>>>>>><Context-cannotReturn.st> >>>>>>> >>>>>>> >>>>>>>-- >>>>>>>_,,,^..^,,,_ >>>>>>>best, Eliot >>>>>><ProcessTest-testResumeAfterBCR.st>
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